The invention relates to conductive polymer composite structures and methods for their manufacture. More particularly, the invention relates to injection or compression molded conductive polymer compositions and techniques for their manufacture.
Polymers can have fundamentally different structures, and one distinguishes between thermoset and thermoplastic polymers. Thermoset polymers are cured over time using heat and chemicals to form chemical bonds. This process is not reversible, and the resulting structure is permanent, unless hydrolysis, chemical attack or oxidation at high temperatures degrades the structural bonds. Thermoplastics, on the other hand, can be formed with a simple melting and cooling process. Upon cooling, the polymer solidifies into the desired shape. Of particular importance, parts containing thermoplastic polymer have the advantage of being recyclable, and can be manufactured with overall cycle times generally below 30 seconds. Highly conductive polymer composite structures with complex geometry are being developed and manufactured for use as corrosion resistant structures in electronic, electrochemical, thermal and thermoelectric device applications, such as collector plates in PEM fuel cells.
Injection molding is a well known method for mass-producing plastic components. Using this method, complex parts can be inexpensively produced with excellent detail and tolerance control. In addition, parts molded from certain plastics have very good resistance to chemical attack and corrosion, albeit at moderate temperatures. The injection molding method of manufacturing has been limited to materials with relatively poor electrical and thermal conductivity. This limitation results from the need for polymer-rich compositions to ensure adequate flow during the molding process; and polymers are poor conductors of heat and electricity. Consequently, the development of low cost and corrosion resistant structures for electronic, electrochemical, thermal and thermoelectric device applications requires innovation in the field of highly conductive polymer composites. A highly conductive polymer composition and a process for molding the composition that incorporates high flow and high electrical and thermal conductivity has been developed by the author et. al., and is described in detail in U.S. patent application Ser. No. 195,307, filed Nov. 18, 1998. This composition, although highly filled, is injection moldable and designed for use in PEM fuel cells; functioning as an electrical conductor, heat transfer material and gas impermeable barrier with resistance to creep, hydrolysis and chemical attack at elevated temperatures. The composition is used to mold collector plates, which can comprise up to 90% by weight of each fuel cell. The constituents of a typical PEM fuel cell are depicted in FIG. 1. Injection molded composite bipolar plates (particularly those containing thermoplastics) develop electrically resistive, polymer rich surface layers during molding, and these layers can affect the performance of fuel cells during operation. If, as in a fuel cell, an electric current is conducted across an interface containing such surface layers, a significant portion of the electric current will be transformed into heat, decreasing the electrical efficiency of the fuel cell. Polymer composites molded according to this inventive molding process are highly conductive compared to prior molded polymer composites. However, this conductivity is restricted due to the higher concentrations of polymer resin at the exterior surfaces of the molded composite structure.
Accordingly, it is an objective of the invention to provide a highly conductive polymer composite structure utilizing injection or compression molding.
It is another object of the invention to further enhance the conductivity of such a molded polymer composite structure.
It is still another object of the invention to utilize a molded highly conductive polymer composite as a collector plate in a fuel cell.
It is yet another object of the invention to provide a molded highly conductive polymer composite collector plate having polymer enhanced channel surfaces, resulting in greater mechanical strength, decreased permeability and porosity, greater resistance to erosion and corrosion, and reduced resistance to flow in the channels.
It is a further object of the invention to provide a method of manufacturing a molded highly conductive polymer composite that utilizes a larger mold cavity, resulting in reduced mold pressure requirements.
It is a further object of the invention to provide a method of manufacturing a molded highly conductive polymer composite having reduced thickness.
It is a still further object of the invention to enhance the conductivity of a molded polymer composite structure utilizing relatively low cost manufacturing processes.
These and other objects of the invention are achieved by a method of fabricating a current collector plate for use in a fuel cell. The method preferably includes the steps of: providing a preferably graphite filled polymer composition shaped as a current collector plate having land areas on opposing surfaces, and removing a layer of said composition from at least one of said land areas. The composition after molding has a gradient of increasing concentration of binder, such as a polymer binder toward said land areas. After the layer removal, new land areas having reduced concentrations of polymer are provided. The highly filled polymer can be molded utilizing injection molding, compression molding or a combination of both. The composition can include a variety of polymers, but is preferably a thermoplastic. The thermoplastic is preferably a liquid crystal polymer. The layer removal is preferably performed using relatively low cost manufacturing processes such as machining, sanding or surface grinding.
The thickness of the layer to be removed can be determined by consideration of a number of factors. It is desirable that the thickness is sufficiently large to remove areas of high polymer concentration. It may be further desirable to remove an even greater thickness to improve the molding process. The removed layer should be between 0.001 and 0.5 cm thick, and is preferably in the range of 0.015 and 0.06 cm thick.
The invention is also directed to an improved fuel cell collector plate resulting from the above described manufacturing process. The improved fuel cell collector plate preferably includes a graphite-filled polymer composition having opposing planar surfaces spaced by a plate thickness. Initially, after molding, the composition has a gradient of increasing concentration of polymer toward said planar surfaces. The planar surfaces have channels and land areas outside the channels. After the layer removal on the land areas, the collector plate provides higher concentrations of conductive graphite filler in the land areas, while maintaining polymer rich surfaces within the channels. These polymer enhanced channel surfaces provide a number of benefits within the channels, such as improved mechanical strength and reduced permeability and porosity.
According to another aspect of the invention, the thickness of the removed surface layer can be predetermined by a test to identify the thickness at which the rate of conductivity change converts from a rapid change to a moderate one.
Another advantage provided by the layer removal process of the invention is that the composite structure can be molded to an initial thickness that is larger than the thickness of the final product, such as a collector plate. In injection molding, the required injection pressure is generally proportional to the ratio of the flow length to cross sectional area of the mold cavity. Thus, with an increased mold cavity thickness, a number of benefits can be achieved. The required injection pressure can be decreased because of the increased cross sectional area. Alternatively, a larger flow length and thus a larger part, can be molded at a given injection pressure. When combined with the layer removal process of the invention, a final collector plate product having an increased aspect ratio can be obtained.
The method of the invention results in conductive polymer compositions particularly for use as fuel cell collector plates having improved functionality. The improved functionality includes very high electrical and thermal conductivity, high strength and low hydrogen permeability in thin sections, and excellent resistance to erosion-corrosion at elevated temperature.